OAR 340-041-0053
Mixing Zones


(1)

The Department may allow a designated portion of a receiving water to serve as a zone of dilution for wastewaters and receiving waters to mix thoroughly and this zone will be defined as a mixing zone;

(2)

The Department may suspend all or part of the water quality standards, or set less restrictive standards in the defined mixing zone, provided that the following conditions are met:

(a)

A point source for which the mixing zone is established may not cause or significantly contribute to any of the following:

(A)

Materials in concentrations that will cause acute toxicity to aquatic life as measured by a Department approved bioassay method. Acute toxicity is lethal to aquatic life as measured by a significant difference in lethal concentration between the control and 100 percent effluent in an acute bioassay test. Lethality in 100 percent effluent may be allowed due to ammonia and chlorine only when it is demonstrated on a case-by-case basis that immediate dilution of the effluent within the mixing zone reduces toxicity below lethal concentrations. The Department may on a case-by-case basis establish a zone of immediate dilution if appropriate for other parameters;

(B)

Materials that will settle to form objectionable deposits;

(C)

Floating debris, oil, scum, or other materials that cause nuisance conditions; and

(D)

Substances in concentrations that produce deleterious amounts of fungal or bacterial growths.

(b)

A point source for which the mixing zone is established may not cause or significantly contribute to any of the following conditions outside the boundary of the mixing zone:

(A)

Materials in concentrations that will cause chronic (sublethal) toxicity. Chronic toxicity is measured as the concentration that causes long-term sublethal effects, such as significantly impaired growth or reproduction in aquatic organisms, during a testing period based on test species life cycle. Procedures and end points will be specified by the Department in wastewater discharge permits;

(B)

Exceedances of any other water quality standards under normal annual low flow conditions.

(c)

The limits of the mixing zone must be described in the wastewater discharge permit. In determining the location, surface area, and volume of a mixing zone area, the Department may use appropriate mixing zone guidelines to assess the biological, physical, and chemical character of receiving waters, effluent, and the most appropriate placement of the outfall, to protect instream water quality, public health, and other beneficial uses. Based on receiving water and effluent characteristics, the Department will define a mixing zone in the immediate area of a wastewater discharge to:

(A)

Be as small as feasible;

(B)

Avoid overlap with any other mixing zones to the extent possible and be less than the total stream width as necessary to allow passage of fish and other aquatic organisms;

(C)

Minimize adverse effects on the indigenous biological community, especially when species are present that warrant special protection for their economic importance, tribal significance, ecological uniqueness, or other similar reasons determined by the Department and does not block the free passage of aquatic life;

(D)

Not threaten public health;

(E)

Minimize adverse effects on other designated beneficial uses outside the mixing zone.

(d)

Temperature Thermal Plume Limitations. Temperature mixing zones and effluent limits authorized under 340-041-0028 (Temperature)(12)(b) will be established to prevent or minimize the following adverse effects to salmonids inside the mixing zone:

(A)

Impairment of an active salmonid spawning area where spawning redds are located or likely to be located. This adverse effect is prevented or minimized by limiting potential fish exposure to temperatures of 13 degrees Celsius (55.4 Fahrenheit) or more for salmon and steelhead, and 9 degrees Celsius (48 degrees Fahrenheit) or more for bull trout;

(B)

Acute impairment or instantaneous lethality is prevented or minimized by limiting potential fish exposure to temperatures of 32.0 degrees Celsius (89.6 degrees Fahrenheit) or more to less than 2 seconds);

(C)

Thermal shock caused by a sudden increase in water temperature is prevented or minimized by limiting potential fish exposure to temperatures of 25.0 degrees Celsius (77.0 degrees Fahrenheit) or more to less than 5 percent of the cross section of 100 percent of the 7Q10 low flow of the water body; the Department may develop additional exposure timing restrictions to prevent thermal shock; and

(D)

Unless the ambient temperature is 21.0 degrees of greater, migration blockage is prevented or minimized by limiting potential fish exposure to temperatures of 21.0 degrees Celsius (69.8 degrees Fahrenheit) or more to less than 25 percent of the cross section of 100 percent of the 7Q10 low flow of the water body.

(e)

The Department may request the applicant of a permitted discharge for which a mixing zone is required, to submit all information necessary to define a mixing zone, such as:

(A)

Type of operation to be conducted;

(B)

Characteristics of effluent flow rates and composition;

(C)

Characteristics of low flows of receiving waters;

(D)

Description of potential environmental effects;

(E)

Proposed design for outfall structures.

(f)

The Department may, as necessary, require mixing zone monitoring studies and/or bioassays to be conducted to evaluate water quality or biological status within and outside the mixing zone boundary;

(g)

The Department may change mixing zone limits or require the relocation of an outfall, if it determines that the water quality within the mixing zone adversely affects any existing beneficial uses in the receiving waters.
340‑041‑0001
Preface
340‑041‑0002
Definitions
340‑041‑0004
Antidegradation
340‑041‑0007
Statewide Narrative Criteria
340‑041‑0009
Bacteria
340‑041‑0011
Biocriteria
340‑041‑0016
Dissolved Oxygen
340‑041‑0019
Nuisance Phytoplankton Growth
340‑041‑0021
pH
340‑041‑0028
Temperature
340‑041‑0031
Total Dissolved Gas
340‑041‑0032
Total Dissolved Solids (TDS)
340‑041‑0033
Toxic Substances
340‑041‑0036
Turbidity
340‑041‑0046
Water Quality Limited Waters
340‑041‑0053
Mixing Zones
340‑041‑0057
Implementation at Domestic Wastewater Treatment Works
340‑041‑0059
Variances
340‑041‑0061
Other Implementation of Water Quality Criteria
340‑041‑0101
Basin-Specific Criteria (Main Stem Columbia River): Beneficial Uses to Be Protected in the Main Stem Columbia River
340‑041‑0103
Basin-Specific Criteria (Main Stem Columbia River): Approved TMDLs in the Basin
340‑041‑0104
Basin-Specific Criteria (Main Stem Columbia River): Water Quality Standards and Policies Specific to the Main Stem Columbia River
340‑041‑0121
Basin-Specific Criteria (Main Stem Snake River): Beneficial Uses to Be Protected in the Main Stem Snake River
340‑041‑0122
Basin-Specific Criteria (Main Stem Snake River): Approved TMDLs in the Basin
340‑041‑0124
Basin-Specific Criteria (Main Stem Snake River): Water Quality Standards and Policies Specific to the Main Stem Snake River
340‑041‑0130
Basin-Specific Criteria (Deschutes): Beneficial Uses to Be Protected in the Deschutes Basin
340‑041‑0133
Basin-Specific Criteria (Deschutes): Approved TMDLs in the Basin:
340‑041‑0135
Basin-Specific Criteria (Deschutes): Water Quality Standards and Policies for this Basin
340‑041‑0140
Basin-Specific Criteria (Goose and Summer Lakes): Beneficial Uses to be Protected in Goose and Summer Lake Basins
340‑041‑0143
Basin-Specific Criteria (Goose and Summer Lakes): Approved TMDLs in the Basin:
340‑041‑0145
Basin-Specific Criteria (Goose and Summer Lakes): Water Quality Standards and Policies for this Basin
340‑041‑0151
Basin-Specific Criteria (Grande Ronde): Beneficial Uses to Be Protected in the Grande Ronde Basin
340‑041‑0154
Basin-Specific Criteria (Grande Ronde): Approved TMDLs in the Basin:
340‑041‑0156
Basin-Specific Criteria (Grande Ronde): Water Quality Standards and Policies for this Basin
340‑041‑0160
Basin-Specific Criteria (Hood): Beneficial Uses to Be Protected in the Hood Basin
340‑041‑0164
Basin-Specific Criteria (Hood): Approved TMDLs in the Basin:
340‑041‑0165
Basin-Specific Criteria (Hood): Water Quality Standards and Policies for this Basin
340‑041‑0170
Basin-Specific Criteria (John Day): Beneficial Uses to Be Protected in the John Day Basin
340‑041‑0174
Basin-Specific Criteria (John Day): Approved TMDLs in the Basin:
340‑041‑0175
Basin-Specific Criteria (John Day): Water Quality Standards and Policies for this Basin
340‑041‑0180
Basin-Specific Criteria (Klamath): Beneficial Uses to Be Protected in the Klamath Basin
340‑041‑0184
Basin-Specific Criteria (Klamath): Approved TMDLs in the Basin:
340‑041‑0185
Basin-Specific Criteria (Klamath): Water Quality Standards and Policies for this Basin
340‑041‑0190
Basin-Specific Criteria (Malheur Lake): Beneficial Uses to Be Protected in the Malheur Lake Basin
340‑041‑0194
Basin-Specific Criteria (Malheur Lake): Approved TMDLs in the Basin:
340‑041‑0195
Basin-Specific Criteria (Malheur Lake): Water Quality Standards and Policies for this Basin
340‑041‑0201
Basin-Specific Criteria (Malheur River):Beneficial Uses to Be Protected in the Malheur River Basin
340‑041‑0204
Basin-Specific Criteria (Malheur River): Approved TMDLs in the Basin:
340‑041‑0207
Basin-Specific Criteria (Malheur River): Water Quality Standards and Policies for this Basin
340‑041‑0220
Basin-Specific Criteria (Mid Coast Basin): Beneficial Uses to Be Protected in the Mid Coast Basin
340‑041‑0224
Basin-Specific Criteria (Mid Coast Basin): Approved TMDLs in the Basin:
340‑041‑0225
Basin-Specific Criteria (Mid Coast Basin): Water Quality Standards and Policies for this Basin
340‑041‑0230
Basin-Specific Criteria (North Coast): Beneficial Uses to Be Protected in the North Coast Basin
340‑041‑0234
Basin-Specific Criteria (North Coast): Approved TMDLs in the Basin:
340‑041‑0235
Basin-Specific Criteria (North Coast): Water Quality Standards and Policies for this Basin
340‑041‑0250
Basin-Specific Criteria (Owyhee): Beneficial Uses to Be Protected in the Owyhee Basin
340‑041‑0254
Basin-Specific Criteria (Owyhee): Approved TMDLs in the Basin:
340‑041‑0256
Basin-Specific Criteria (Owyhee): Water Quality Standards and Policies for this Basin
340‑041‑0260
Basin-Specific Criteria (Powder/Burnt): Beneficial Uses to Be Protected in the Powder/Burnt Basins
340‑041‑0264
Basin-Specific Criteria (Powder/Burnt): Approved TMDLs in the Basin:
340‑041‑0265
Basin-Specific Criteria (Powder/Burnt): Water Quality Standards and Policies for this Basin
340‑041‑0271
Basin-Specific Criteria (Rogue): Beneficial Uses to Be Protected in the Rogue Basin
340‑041‑0274
Basin-Specific Criteria (Rogue): Approved TMDLs in the Basin:
340‑041‑0275
Basin-Specific Criteria (Rogue): Water Quality Standards and Policies for this Basin
340‑041‑0286
Basin-Specific Criteria (Sandy Basin): Beneficial Uses to Be Protected in the Sandy Basin
340‑041‑0289
Basin-Specific Criteria (Sandy Basin): Approved TMDLs in the Basin:
340‑041‑0290
Basin-Specific Criteria (Sandy Basin): Water Quality Standards and Policies for this Basin
340‑041‑0300
Basin-Specific Criteria (South Coast): Beneficial Uses to Be Protected in the South Coast Basin
340‑041‑0304
Basin-Specific Criteria (South Coast): Approved TMDLs in the Basin
340‑041‑0305
Water Quality Standards and Policies for South Coast Basin
340‑041‑0310
Basin-Specific Criteria (Umatilla): Beneficial Uses to Be Protected in the Umatilla Basin
340‑041‑0314
Basin-Specific Criteria (Umatilla): Approved TMDLs in the Basin:
340‑041‑0315
Basin-Specific Criteria (Umatilla): Water Quality Standards and Policies for this Basin
340‑041‑0320
Basin-Specific Criteria (Umpqua Basin): Beneficial Uses to Be Protected in the Umpqua Basin
340‑041‑0324
Basin-Specific Criteria (Umpqua Basin): Approved TMDLs in the Basin:
340‑041‑0326
Basin-Specific Criteria (Umpqua Basin): Water Quality Standards and Policies for this Basin
340‑041‑0330
Basin-Specific Criteria (Walla Walla): Beneficial Uses to Be Protected in the Walla Walla Basin
340‑041‑0334
Basin-Specific Criteria (Walla Walla): Approved TMDLs in the Basin:
340‑041‑0336
Basin-Specific Criteria (Walla Walla): Water Quality Standards and Policies for this Basin
340‑041‑0340
Basin-Specific Criteria (Willamette): Beneficial Uses to Be Protected in the Willamette Basin
340‑041‑0344
Basin-Specific Criteria (Willamette): Approved TMDLs in the Basin:
340‑041‑0345
Basin-Specific Criteria (Willamette): Water Quality Standards and Policies for this Basin
340‑041‑0350
The Three Basin Rule: Clackamas, McKenzie (above RM 15) & the North Santiam
340‑041‑8033
Division 41 Tables and Figures
Last Updated

Jun. 8, 2021

Rule 340-041-0053’s source at or​.us